Designing a Highly Active Metal‐Free Oxygen Reduction Catalyst in Membrane Electrode Assemblies for Alkaline Fuel Cells: Effects of Pore Size and Doping‐Site Position

Abstract

To promote the oxygen reduction reaction of metal-free catalysts, the introduction of porous structure is considered as a desirable approach because the structure can enhance mass transport and host many catalytic active sites. However, most of the previous studies reported only half-cell characterization; therefore, studies on membrane electrode assembly (MEA) are still insufficient. Furthermore, the effect of doping-site position in the structure has not been investigated. Here, we report the synthesis of highly active metal-free catalysts in MEAs by controlling pore size and doping-site position. Both influence the accessibility of reactants to doping sites, which affects utilization of doping sites and mass-transport properties. Finally, an N,P-codoped ordered mesoporous carbon with a large pore size and precisely controlled doping-site position showed a remarkable on-set potential and produced 70% of the maximum power density obtained using Pt/C.

Quantified mass transfer and superior antiflooding performance of ordered macro‐mesoporous electrocatalysts

“…When flooding happens in the catalyst layer (Figure 1), the accumulation of water in the porous channels directly results in interruption of O 2 supply to the active sites and termination of ORR in the flooded region, thus heavily changes the overpotentials and reaction kinetics of the fuel cell. 17 Until now, a range of porous electrochemical catalysts with macropores, mesopores, micropores and their various combinations in ordered or disordered arrangement have been reported, [18][19][20][21][22][23][24][25][26][27][28][29] and the present investigations well covered the relationships among the specific surface areas, the pore sizes, the distribution of the active sites, and the activities. 17 Until now, a range of porous electrochemical catalysts with macropores, mesopores, micropores and their various combinations in ordered or disordered arrangement have been reported, [18][19][20][21][22][23][24][25][26][27][28][29] and the present investigations well covered the relationships among the specific surface areas, the pore sizes, the distribution of the active sites, and the activities.…”

Tree‐Bark‐Shaped N‐Doped Porous Carbon Anode for Hydrazine Fuel Cells

“…Figure 2s hows the results on the surface area and pore size distribution of the NDCs from the N 2 adsorption/ desorption (ads/des) measurements. [16] CNF has the lowest surface area, as calculated using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods,m easured as 11.77 and 8.17 m 2 g À1 ,r espectively ( Figure 2a). The low and similar values of the BET and BJH surface areas imply that CNF has an on-porous structure with as mooth fibrous morphology,w hich is in good agreement with the TEM result (Figure 1a).…”

“…CNF shows the lowest power performance of 10.3 mW cm À2 ,w hile Va p-PM-CNF exhibits the highest power performance of 127.5 mW cm À2 ,w hich are in good agreement with the halfcell result (Figure 4b). According to the previous result that evaluated the effect of the pore size distribution on the cell performance, [16] the active sites in the mesoporous structures,n ot those in the microporous structures,m ainly contribute to the cell performance. Considering our hypothesis that the activity of hydrazine oxidation is proportional to the amount of exposed N-C structures in the pore structures,t his disagreement between the results of the half cell and single cell could be explained by the different accessibility of hydrazine molecules to the active sites in the practical operation of the DHFC.Unlike the half-cell system, it is difficult for hydrazine to permeate through the micropores of ac atalyst in the practical operation of asingle cell.…”

“…[12][13][14][15][16] In addition, for the first time,the performances of the DHFCs with these metal-free catalysts as the anodes are evaluated. Thep ore size in/on the NDCs is selectively controlled by poly(methyl methacrylate) (PMMA) and water vapor treatments to fabricate the following materials:N -doped carbon nanofiber (CNF), PMMA-treated CNF (PM-CNF), water-vapor-treated CNF (Vap-CNF), and both PMMA-and water-vapor-treated CNF (Vap-PM-CNF).…”

Cobalt‐Embedded Nitrogen‐Doped Carbon Nanotubes as High‐Performance Bifunctional Oxygen Catalysts

“…Compared with the other catalysts, Ni@NCNT-700 had the highestt otal Nc ontent,w hich suggested that the total Ncontent was not the key factor that influencedthe performanceofbifunctional oxygencatalysts,althoughe volution of the pyridinic-N content was related to performance.T his might be explained by pyridinic-N species providing the main active sites,w hich could reduce the adsorption energy of O 2 and improve the onset potential for the ORR,a sr eported previously. [16,20,22,[39][40][41][42] Figure 5d shows the Tafel plots of Co-, Fe-, and Ni-containing CNT catalysts towards the ORR.F e@NCNT-700 had ah igh onset potential (0.91 V) and poor half-wave potential in Figure 2c.Ahigh onset potential is usuallyr elated to sufficient pyridinic N, whereast he half-wave potential might be due to the worst Tafel slope (80.5 mV dec À1 ), representing poor kinetic processes,w hich was attributed to the absence of graphitic N. [8,12,40] Moreover, although Ni@NCNT-700 had the best Tafel slope (37.0 mV dec À1 ), which was attributedt ot he abundance of graphitic N, it still had the poorest ORR performance,w hich might be related to the presenceo fm etal-N species andconsequently the loss of pyridinic N. Herein, Co@NCNT-700, with suitable nitrogen chemical states (pyridinic and graphitic N), gave the best ORR performance.H owever, in the OER,N i@NCNT-700 displayed as imilar onset potential to that of Co@NCNT-700, which suggested that the metal element played an importantr ole in the OER process,r ather than the chemical state of N. Therefore,b oth nitrogen chemical state and metal element could significantly influence the electrochemical performanceo f these bifunctional oxygenc atalysts.T he clear change in the nitrogen chemical state of Ni@NCNT-700 helped us to clarify the confusion, to some extent, of why many kinds of Ni-containing nitrogen-doped carbon materials had huge differences in electrochemical processes in comparison with Co-/Fecontainingc atalysts,e specially for the ORR.O ur work is beneficial to the developmento fn itrogen-doped carbon materials as bifunctional ORR/OERc atalysts.…”